17/35

AN628 APPLICATION NOTE

Pin 17. ROSC (Oscillator resistor). An external resistor connected to ground, programs the charge and the dis-

(see fig.18a/b)

To set the charge current, the relation is:

The discharge current is defined by:

Ω.

Pin 18. COSC (Oscillator capacitor). An external capacitor (see fig 18a/b), connected between this pin and

The period T is defined by:

the switching frequency is:

See also Fig. 19

I

c

10

1.28V

R

OS C

----------------

⋅

≈

I

c

200

1.28V

R

OSC

----------------

⋅

≈

Tt

r

t

f

+

V

srp

C

OSC

1

I

C

-----

1

I

d

----

+





⋅

==

f

sw

1

T

---

2.44

R

OSC

C

OS C

⋅

------------------------------------

≈

=

Figure 19. Oscillator Diagram

Pin 19. VCC (Supply voltage input). The very low cur-

rent consumption feature before the turn-on thresh-

old is reached. The undervoltage circuitry, with the

threshold hysteresis of 5.5V typ. (see also pin 15)

and an internal clamp at 25V (typ.) ensure the IC

safety operation.

Pin 20. GDRV (Gate driver output). This output is in-

ternally clamped to 15V (see Fig. 20), to avoid ageing

problems of the gate oxide. The output driver is nor-

mally connected to the gate of the power device

through a resistor (say 5 to 50 Ohm) to avoid over-

shoot and to control the dI/dt of the switch.

POWER SECTION DESIGN

Booster Inductor

The Boost Inductor design involves various parame-

ters to be handled and there are different approaches

to define them.

In continuous mode operation, the energy stored in

the boost inductor in each switching cycle, is not

completely transferred to the output (bulk) capacitor.

A quantity of energy is stored in the magnetic circuit,

reducing in this way the input current ripple. This min-

imizes the line noise and reduces the input filter size

(see fig.21).